The present invention relates to a UHF/VHF tuner and, more particularly, to a UHF/VHF tuner in which complication in structure and increase in manufacturing costs are suppressed and broadcast receiving performance is improved by reducing variation in the reception sensitivity according to frequencies when a broadcast in the VHF band is received.
Hitherto, in a UHF/VHF tuner for U.S.A., an RF circuit for UHF and an RF circuit for VHF are separately provided and a frequency converting stage of each of the UHF and VHF circuits is constructed by one mixer and one local oscillator.
FIG. 3 is a block diagram showing an example of the construction of main components of a known UHF/VHF tuner. FIG. 4 is a block diagram in which an example of the construction of frequency converting stages of the UHF/VHF tuner shown in FIG. 3 is illustrated in a partial circuit diagram. Component elements which are the same as those shown in FIG. 3 are designated by the same reference numerals.
As shown in FIG. 3, the known UHF/VHF tuner comprises a UHF antenna (ANT) tuning circuit 31, a UHF high frequency amplifier (RFAMP) 32, a UHF interstage tuning circuit 33, a UHF mixer (MIX) 34, a UHF local oscillator (OSC) 35, a VHF antenna (ANT) tuning circuit 36, a VHF high frequency amplifier (RFAMP) 37, a VHF interstage tuning circuit 38, a VHF mixer (MIX) 39, a VHF local oscillator (OSC),40, an intermediate frequency amplifier (IFAMP) 41, a reception signal input terminal 42, and an intermediate frequency signal output terminal 43. The component elements 31 to 43 are connected as shown in FIG. 3.
As shown in FIG. 4, the UHF local oscillator 35 is constructed by a first circuit part 35(1) including a transistor for oscillation integrated in an integrated circuit 44 and a second circuit part 35(2) externally attached to the integrated circuit 44. The VHF local oscillator 40 is also constructed by a first circuit part 40(1) including a transistor for oscillation integrated in the integrated circuit 44 and a second circuit part 40(2) externally attached to the integrated circuit 44.
The second circuit part 35(2) of the UHF local oscillator 35 comprises a varactor diode 351, an inductor 352, capacitors 353 to 358, and resistors 359 and 3510. Those elements 351 to 3510 are connected as shown in FIG. 4. The second circuit part 40(2) of the VHF local oscillator 40 comprises varactor diodes 401 to 403, inductors 404 to 406, capacitors 407 to 4016, a switching diode 4017, and resistors 4018 to 4023. Those elements 401 to 4023 are connected as shown in FIG. 4. Further, a PLL 45 is arranged in the integrated circuit 44. Band switching terminals BS2 and BS3 in the PLL 45 are connected as shown in FIG. 4 and tuning voltage terminals TU (U) and TU (V) in the PLL 45 are connected also as shown in FIG. 4.
The known UHF/VHF tuner having the above construction operates as follows.
In case of receiving a broadcast in the UHF band, a reception signal is supplied to the reception signal input terminal 42. From the reception signal, only a reception signal of a desired channel is selected by the UHF antenna tuning circuit 31 and is amplified to a predetermined level by the UHF amplifying circuit 32. From the amplified reception signal, only a reception signal of the desired channel is further selected by the UHF interstage tuning circuit 33 and is supplied to the UHF mixer 34. The UHF mixer 34 mixes the frequencies of the reception signal selected by the UHF interstage tuning circuit 33 and a local oscillation signal generated by the UHF local oscillator 35. The intermediate frequency amplifier 41 selectively amplifies an intermediate frequency signal indicative of the frequency as the difference between the reception signal and the local oscillation signal and leads the resultant signal to the intermediate frequency signal output terminal 43.
In this instance, in the UHF local oscillator 35, the oscillation frequency is set by values such as capacitance values of the varactor diode 351 and the capacitors 353 and 354 and the inductance value of the inductor 352. Among them, the capacitance value of the varactor diode 351 is set by an output tuning voltage of the tuning voltage terminal TU (U) in the PLL 45 supplied to the varactor diode 351 via the resistor 3510.
In the event of receiving a broadcast in the VHF band, a reception signal is supplied to the reception signal input terminal 42. From the reception signal, only a reception signal of a desired channel is selected by the VHF antenna tuning circuit 36 and is amplified by the VHF amplifying circuit 37 to a predetermined level. From the amplified reception signal, only a reception signal of the desired channel is further selected by the VHF interstage tuning circuit 38 and is supplied to the VHF mixer 39. The VHF mixer 39 mixes the frequencies of the reception signal selected by the VHF interstage tuning circuit 38 and the local oscillation signal generated by the VHF local oscillator 40. The intermediate frequency amplifier 41 selectively amplifies an intermediate frequency signal indicative of the frequency as the difference between the reception signal and the local oscillation signal and leads the resultant signal to the intermediate frequency signal output terminal 43.
In this instance, in the VHF local oscillator 40, when a high-band channel is selected, the oscillation frequency is set by values such as capacitance values of the varactor diode 40, and the capacitors 407, 408, and 4013, and the inductance value of the inductor 404. On the other hand, when a low-band channel is selected, the oscillation frequency is set by values such as capacitance values of the varactor diode 401, and the capacitors 407 to 4010, and 4013, and the inductance values of the inductors 404 and 405. When either one of the channels is selected, the capacitance value of the varactor diode 401 is set by an output tuning voltage of the tuning voltage terminal TU (V) in the PLL 45 supplied to the varactor diode 401 via the resistor 4018. When the high band is selected, a positive voltage is applied to the band switching terminal BS2 in the PLL 45. The positive voltage is supplied to the anode of the switching diode 4017 through the resistor 4020 and the inductor 405 to thereby turn on the switching diode 4017. Consequently, the inductor 405 and the capacitors 409 and 4010 are bypassed from an oscillation frequency determining circuit in the VHF local oscillator 40. The oscillation frequency determining circuit is constructed by the varactor diode 401, the capacitors 407, 408, and 4013 and the inductor 404. On the other hand, when the low band is selected, the positive voltage is applied to the band switching terminal BS3 in the PLL 45. The positive voltage is supplied to the cathode of the switching diode 4017 via the resistor 4022 to thereby turn off the switching diode 4017. Consequently, the inductor 405 and the capacitors 409 and 4010 construct the oscillation frequency determining circuit of the VHF local oscillator 40 together with the varactor diode 401, the capacitors 407, 408, and 4013, and the inductor 404.
According to the known UHF/VHF tuner, however, when the broadcast in the VHF band is received, it is difficult for the first circuit part 40(1) including the transistor for oscillation to generate a sufficient local oscillation signal in a wide frequency range of both of the high and low bands. In the case where a broadcast of the high frequency side of the high band and/or the low frequency side of the low band is selected, the level of the local oscillation signal becomes lower than that of the local oscillation signal when a broadcast of other frequency is selected and there is a problem such that the reception sensitivity largely varies according to the frequency of the broadcast.
The invention is to solve the problem and its object is to provide a UHF/VHF tuner in which complication in structure and increase in manufacturing costs are suppressed and broadcast receiving performance is improved by reducing the variation in the reception sensitivity according to the frequency when a broadcast in the VHF band is received.
In order to achieve the object, the UHF/VHF tuner according to the invention comprises means using both of a frequency converting stage having one mixer and one UHF local oscillator for reception of the broadcast in the UHF band and a frequency converting stage having one mixer, a VHF high-band local oscillator and a VHF low-band local oscillator for reception of the broadcast in the VHF band.
According to the means, when the broadcast in the VHF band is received, the VHF high-band local oscillator is used to receive the broadcast of a high-band channel, the VHF low-band local oscillator is used to receive the broadcast of a low-band channel, and a mixer for mixing the frequencies of the reception signal and the local oscillation signal is commonly used for reception of broadcasts of the high and low band channels. Consequently, the reception sensitivity does not largely vary according to the broadcast frequency in the event of receiving the broadcast in the VHF band. Moreover, since only one VHF local oscillator is added as compared with a known tuner of this kind, the construction is not complicated and increase in the manufacturing costs is a little.
In an embodiment of the invention, a UHF/VHF tuner receives both of a broadcast in the UHF band and a broadcast in the VHF band. One mixer and one local oscillator are used for the broadcast in the UHF band and one mixer and two local oscillators which are switched in correspondence to the broadcast reception band are used for the broadcast in the VHF band
In a preferred embodiment of the invention, the two local oscillators of the UHF/VHF tuner can be switched by an output of a band decoder in a PLL.
According to the embodiments of the invention, in the event of receiving a broadcast in the VHF band, a VHF high-band local oscillator is used for receiving a high-band channel broadcast and a VHF low-band local oscillator is used for receiving a low-band channel broadcast. Consequently, the signal level difference between the oscillation frequencies in the bands is reduced and the mixer for mixing the frequencies of the reception signal and the local oscillation signal is used for reception of both of the broadcasts in the high-band and low-band channels. When the broadcast in the VHF band is received, the reception sensitivity does not largely vary according to the broadcast frequency. The broadcast reception performance can be therefore improved. Moreover, when it is compared with a known tuner of this kind, only one VHF local oscillator is just added. Thus, the construction is not complicated and increase in the manufacturing costs is a little.